This invention relates to a method of making thick film sensors on a stainless steel diaphragm for detecting the pressure and temperature of a high pressure and/or corrosive fluid.
In automotive control applications, it is frequently necessary to measure the pressure of certain high pressure and/or corrosive fluids, such as engine crankcase oil, transmission fluid, engine coolant, or brake fluid. A proven approach to pressure sensing in such an environment involves the use of a pressure port that attaches to the pressure vessel by a threaded fitting, and a stainless steel diaphragm welded or otherwise secured to the pressure port. The inboard surface of the diaphragm is in contact with the pressurized fluid, and the interface between the diaphragm and the pressure port provides a leak-proof seal between the fluid and the sensor housing. Typically, resistive elements are formed on the outboard surface of the diaphragm for sensing diaphragm stress induced by the fluid pressure. See, for example, the U.S. Pat. No. 5,629,486 to Viduya et al., issued on May 13, 1997, and U.S. Pat. No. 5,867,886 to Ratell et al., issued on Feb. 9, 1999, both of which are assigned to Delphi Technologies, Inc., and incorporated herein by reference. In U.S. Pat. No. 5,629,486, the outboard surface of the diaphragm is covered with an insulating coating and a conductive film, and the resistive elements are formed by scribing the conductive film. In U.S. Pat. No. 5,867,886, thick-film processing is used to cover the outboard surface of the diaphragm with a dielectric, and then to add various patterned conductive and piezo-resistive layers. In both of these patents, at least one compensation IC is also mounted on the outboard surface of the diaphragm (typically, opposite a central mass of the diaphragm, as in the U.S. Pat. No. 5,629,486) and wire-bonded to bond sites adjacent the resistive elements.
In applications where it is also necessary to measure the temperature of the fluid, it has been proposed to modify the pressure port to include a special chamber for receiving a discrete temperature-responisive element. See, for example, the U.S. Pat. Nos. 6,267,010; 6,003,379; and 5,948,989, which disclose the use of a thermistor encased in a portion of the pressure sensor housing. However, such an approach will be cost-prohibitive in many applications, and it is unclear whether the thermal mass of the pressure sensor housing will allow the thermistor to promptly respond to changes in the temperature of the fluid. Accordingly, what is needed is a more reliable and cost effective way of measuring both the pressure and temperature of a high pressure and/or corrosive fluid.
The present invention is directed to a method of making an improved and low-cost metal diaphragm sensor that integrates both pressure and temperature sensing in a single sensor assembly. According to the invention, thick-film processing is used to form a circuit including stress and temperature sensitive elements on the outboard or exposed surface of a thin metal diaphragm separating the circuit from a pressurized fluid. Only a thin layer of dielectric separates the stress and temperature sensitive elements from the diaphragm surface. The stress sensitive elements respond to mechanical stressing of the diaphragm due to the presence of the pressurized fluid, while the temperature sensitive element responds to the temperature of the pressurized fluid. The thermal capacity of the fluid greatly exceeds that of the diaphragm, so that the temperature responsive characteristic of the temperature sensitive element accurately reflects the temperature of the pressurized fluid.